Electrode connection monitoring

ABSTRACT

The present invention relates to an apparatus and a method for measuring muscular activity, in particular in relation to bruxism, and providing an electrical stimulation in response to the measured muscular activity through electrodes applied to the skin of an individual, wherein the quality of the connectivity of the electrodes to the skin is monitored and the electrical stimulation signal is changed based on said quality measurement.

FIELD OF INVENTION

The present invention relates to an apparatus and a method for use inanalysing EMG signals. The apparatus provides for monitoring the qualityof the connection of EMG electrodes to the source of said signals, whichis normally the skin.

BACKGROUND OF INVENTION

In conducting the measurement of EMG signals, electrodes are placed oradhered onto the skin, usually with a layer of electrically conductinggel between the skin surface and the electrode proper. The quality ofthe signals received will depend on the quality of the connection madebetween the skin and the electrode, and this may not be easy for theuser to ascertain. If the connection is poor, the sensing of particularsignals may fail and actions that should be triggered in response toparticular conditions may not take place. In Functional ElectricalStimulation systems, electric currents are impressed on the skin byvoltage applied via skin electrodes either to produce a muscularreaction or to provide a sensory input. If the connection between theelectrode and the skin is poor, this can lead to excessive voltage beingapplied or can lead to a failure to produce the desired effect.

For instance, in WO2004/087258, episodes of bruxism are detected bymonitoring EMG signals received from electrodes on the skin of a userand when such an episode is detected, an mild electric shock is appliedvia the electrodes as a biofeedback signal. In order that the apparatuscan function correctly to detect the bruxism episodes and also to applythe biofeedback signal safely, a good connection between the skin andthe electrodes is required. It is described that the connection can bechecked by applying a voltage between a pair of the electrodes to betested, so as to produce a predetermined current, and measuring thevoltage required, and further that the user is informed, in case ofreduced connection, to adjust the apparatus, such as the strap pressingthe electrodes to the skin.

All patent and non-patent references cited in the application, or in thepresent application, are also hereby incorporated by reference in theirentirety.

SUMMARY OF INVENTION

The present invention relates to an apparatus and a method for measuringmuscular activity, in particular in relation to bruxism, and providingan electrical stimulation in response to the measured muscular activitythrough electrodes applied to the skin of an individual, wherein thequality of the connectivity of the electrodes to the skin is monitoredand the electrical stimulation signal is changed based on said qualitymeasurement.

Accordingly, in a first aspect the invention relates to an apparatuscomprising electrodes adapted for being applied to the skin of anindividual, wherein the apparatus is adapted for receiving andmonitoring electrical signals and comprises respective inputs forreceiving signals from at least a first electrode of said electrodes anda reference electrode, and for providing electrical stimulation to saidindividual, said apparatus comprising

-   -   means for measuring EMG signals from said electrode(s) when the        electrodes are applied to the skin,    -   a source of a monitoring signal having a frequency, an output        for use in applying said monitoring signal to said at least        first electrode,    -   means for determining the received signal of said frequency of        the monitoring signal in signals received at said inputs,    -   means for providing an electrical stimulation feedback signal        upon the detection of EMG signals indicative of a muscular        activity of the individual,    -   wherein the received signal of said frequency of the monitoring        signal in signals received at said inputs is measured for        monitoring the quality of the electrical connection between the        electrodes and the skin, and the feedback signal is modulated if        the quality of the electrical connection between the electrodes        is changed.

Furthermore, the present invention provides a method for monitoring theelectrode connection, wherein the incoming signals relating to themonitoring signals are analysed by a Fourier transform analyser.Accordingly, in a second aspect the invention relates to an apparatusfor receiving and monitoring electrical signals comprising respectiveinputs for receiving signals from at least a first electrode and areference electrode, a Fourier transform analyser for extractingfrequency content information from said signals, a source of amonitoring signal having a frequency, an output for use in applying saidmonitoring signal to a said electrode, and means for determining theamplitude of the said frequency of the monitoring signal in signalsreceived at said inputs as determined by said Fourier transformanalyser.

In yet another aspect the invention relates to a method for amelioratingand/or treating bruxism by applying electrodes as described above to theskin above a facial muscle of said individual, preferably the massetermuscle or the temporalis muscle, applying a stimulating signal, should abruxism event happen, and monitoring the connection between theelectrodes and the skin. Accordingly, the invention relates to A methodfor monitoring activity of the temporal muscle and/or the massetermuscle, in particular activity due to teeth and/or jaw clenching, saidmethod comprising the steps of:

-   -   measuring EMG signals from said electrode(s) when the electrodes        are applied to the skin,    -   applying a monitoring signal having a frequency to said at least        first electrode,    -   determining the received signal of said frequency of the        monitoring signal in signals received, for monitoring the        quality of the electrical connection between the electrodes and        the skin    -   providing an electrical stimulation feedback signal upon the        detection of EMG signals indicative of a muscular activity of        the individual,    -   wherein the feedback signal is modulated if the quality of the        electrical connection between the electrodes is changed.

Furthermore, the invention relates to a method for monitoring electrodeconnections, by receiving and monitoring electrical signals from atleast a first electrode and a reference electrode, extracting frequencycontent information from said signals and performing a Fourier transformanalysis, applying said monitoring signal to a said electrode, anddetermining the amplitude of the said frequency of the monitoring signalin signals received as determined by said Fourier transform analyser.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a block circuit diagram for a bruxism monitoring apparatusof the kind described in WO2004/087258 but modified to incorporateelectrode connection monitoring according to the present invention;

FIG. 2 is a block circuit diagram showing the components of theapparatus of FIG. 1 that are relevant to the electrode connectionmonitoring.

DETAILED DESCRIPTION OF THE INVENTION

As described above the apparatus according to the invention haselectrodes adapted for being applied to the skin of an individual formeasuring EMG as a measure for muscular activity, in particular inrelation to bruxism, and providing an electrical stimulation in responseto the measured muscular activity through the electrodes. The apparatusis in particular useful for measuring muscular activity relating tobruxism during night when the individual is at sleep. Accordingly, theelectrodes are applied to the skin before the individual goes to bed andmust stay on the skin during the sleep until next morning in order to beeffective. The connection of the electrodes may be of good quality atthe start of the sleep, however due to changes in for exampletemperature, pressure and general movements of the individual thequality of the connection may be reduced during the sleep. Thus, thepresent invention provides an apparatus wherein the quality of theconnection may be monitored during use of the apparatus, andfurthermore, that the electrical feedback stimulation signal may bechanged due to changes in the connection, in order to reduce harm to theindividual and maintain efficiency of the apparatus, at least to somedegree.

It will be appreciated that the electrodes may each comprise a solidconductive electrode member provided with a respective patch ofconductive gel in an assembly, in which case it will be the area of thegel that defines the contact area of the electrode rather than the sizeof the electrode member. However, even when a gel is used the connectionbetween electrode and skin may vary depending on a variety of factors,such as described above. The area of an electrode member in contact withsuch a gel patch may be approximately from 5% to 100% of the electrodecontact area (i.e. the gel patch area), in some cases 25% to 75%, e.g.about 50%.

The electrode contact areas may extend along or on a said line adistance of from 1 to 10 mm from one side to an opposite side of theelectrode contact area and may extend transversely of a said line by adistance of from 1 to 10 mm from one side to an opposite side of theelectrode contact area, the maximum distance along said line from anedge of a first said electrode contact area to an opposite edge of thefurthest away of the other two electrode contact areas being not morethan 60 mm, more preferably 50 mm. Preferably, the contact areas of theelectrodes may extend along or on said line a distance of from 3 to 7 mmfrom one side to an opposite side of the electrode contact area.Similarly, the contact areas of the electrodes may extend transverselyof said line by a distance of from 3 to 7 mm from one side to anopposite side of the electrode contact area.

Thus, for instance the electrode contact areas may be circular and ofdiameter from 2-20 mm, more preferably from 5-15 mm, e.g. about 10 mm,and the electrode members may also be circular having a diameter of from1-10 mm, more preferably from 3 to 7 mm, e.g. about 5 mm. The electrodecontact areas and the electrode members may be of other shapes havingequivalent areas to those described above.

Preferably, the apparatus comprises respective inputs for receivingsignals from said first electrode, from a second electrode and from saidreference electrode. Preferably, there are inputs for just the threeelectrodes, although more may be provided. Preferably, there arerespective outputs for applying said monitoring signal to each of thefirst electrode and the second electrode.

The apparatus preferably further comprises respective electrodesconnected to said inputs and to said output(s).

Preferably, the apparatus further comprises circuit means adapted toprovide a preset monitoring signal current between said first electrodeand said reference electrode, and optionally also circuit means adaptedto provide a preset monitoring signal current between said secondelectrode and said reference electrode. The means for determining thereceived signal of said frequency, may be provided to determine andoptionally to record the voltage amplitude of said monitoring signalrequired to achieve said preset current level between said electrodepair(s). Thus, the quality of the connection between the electrodesthrough a material to which they are connected may be determined.

Said electrodes are preferably provided in an electrode assembly inwhich said electrode assembly comprises two or more preferably threeelectrodes mounted on a common substrate in a fixed spatial relationshipone to another. Said electrode assembly is preferably adapted to beapplied to the skin above the masseter or temporalis muscles, ie. of ashape adapted to follow the facial contours most generally met at thevarious individuals to be treated.

Preferably, where first, second and reference electrodes are provided,said reference electrode lies approximately equidistant from said firstand second electrodes.

The apparatus may comprise means for measuring the amplitude within thereceived signals of at least one selected frequency other than that ofsaid monitoring signal.

In a preferred aspect of the practise of the invention, said apparatusis for measuring EMG signals from said electrodes when the electrodesare applied to the skin and the amplitude of the said frequency of themonitoring signal in signals received at said inputs as determined bysaid Fourier transform analyser is measured for monitoring the qualityof the electrical connection between the electrodes and the skin.

Preferably, the apparatus is for functional electrical stimulation(FES). In some FES devices, electrical stimulating pulses are deliveredvia skin mounted electrodes to activate muscle functions. In suchdevices it will be desirable to be able to monitor the connection to theskin of the electrodes by the use of apparatus according to theinvention. However, apparatus according to the invention preferably alsocomprises means for the detection of EMG signals and means for providinga feedback signal upon the detection of EMG signals indicative of amuscular activity.

In particular, said activity may be an episode of bruxism.

In order that the stimulation signal has the desired effect on bruxismbehaviour and yet does not wake the user from sleep, or is adjustable toachieve these ends, a match is needed between the form of the signal andthe form of the electrode assembly. For use with the electrode assemblyconformations described herein, it is preferred that said electricalstimulation signal is a biphasic signal which is initiated at a voltageapplied to the electrode assembly of not more than 10 volts peak to peakand is raised to a maximum peak to peak voltage at a rate of not morethan 500 V/sec, said signal having a duration of not more than 2 sec, asaid maximum voltage of not more than 100 volts peak to peak.

Preferably, said biphasic signal has a pulse width of from 50 μsec to 10msec, more preferably from 50 to 500 μsec, more preferably from 100 to300 μsec, for instance about 150 μsec.

The signal is preferably initiated at a voltage applied to the electrodeassembly of not more than 5 volts peak to peak. The signal preferablyincreases in intensity from its initial value at a rate of not more than350 V/sec, more preferably not more than 250 V/sec, for instance between100 and 250 V/sec, e.g. about 200V/sec.

The duty cycle of the signal may be from 1 to 99%, but is preferably inthe range of from 30 to 70%, suitably about 50%.

The frequencies of muscle activity are normally in the range of from 0to about 350 Hz, whereas the frequencies of bruxism related muscleactivity is normally found in the range of from 250-350 Hz, morespecifically from 268-332 Hz. The frequency of the monitoring signal ispreferably different from the frequencies of muscle activity, and morepreferably the frequency of the monitoring signal is higher than thefrequencies of muscle activity. Accordingly, the frequency of themonitoring signal is preferably above 350 Hz, more preferably above 400Hz, more preferably above 450 Hz, more preferably above 500 Hz, and evenmore preferably about 600 Hz.

Preferably, said EMG signals indicative of the muscular activity areidentified by measuring therein the amplitude of one or more selectedfrequencies different from the frequency of the monitoring signal. Themeasured frequency or frequencies may be selected for maximaldifferentiation between bruxism activity and possibly confoundingmuscular activities such as changes in facial expression, e.g.grimacing, and they may be selected to best suit detection of bruxism inan individual user. Apparatus to which the invention may be applied isdescribed in WO 2004/087258.

If the quality of the connection between the electrode(s) and the skinis reduced, it is preferred that the electrical stimulation signal ischanged based on said quality measurement. For example, if thedetermination of the received signal of said frequency of the monitoringsignal implies a poor connection between then it is preferred that thefeedback stimulation signal is reduced, such as wherein the intensity ofthe feedback stimulation signal is reduced. In a preferred embodimentthe feedback stimulation signal is reduced by half, to avoid burning orskin irritation due to too great a current being passed from oneelectrode to a better connected electrode. In yet another embodiment thefeedback stimulation signal may be blocked.

Accordingly, in one embodiment an interlock may be provided such thatthe means for providing a feedback signal upon the detection of EMGsignals indicative of a muscular activity is prevented from so doing ifthe amplitude of the said frequency of the monitoring signal in signalsreceived at said inputs is not indicative of a sufficiently goodconnection, e.g. is not below a preset threshold level of voltagesufficient to provide a set level of current, or wherein the intensityof the feedback signal is reduced if there is not a sufficiently goodconnection, e.g. if the amplitude of the said frequency of themonitoring signal in signals received at said inputs is not below apreset threshold level of voltage sufficient to provide a set level ofcurrent, e.g. if the amplitude of the said frequency of the monitoringsignal in signals received at said inputs is not below a presetthreshold level of voltage sufficient to provide a set level of current.

Thus, where electrodes are placed on the skin and a feedback stimulationsignal is applied through them, the apparatus according to the inventioncan avoid the feedback signal being provided if the connection to theskin is not sufficiently good, and thus the risk of skin irritation orburning may be avoided.

Preferably, the apparatus comprises a said first electrode, a saidsecond electrode and a said reference electrode, said monitoring signalis applied to said first electrode and to said second electrode and theamplitude of the frequency of the monitoring signal at said referenceelectrode is measured to determine the quality of the connection via theskin between the first electrode and the reference electrode and thequality of the connection via the skin between the second electrode andthe reference electrode.

Suitably, said monitoring signal is applied at a voltage sufficient toproduce a preset current between the first electrode and the referenceelectrode and between the second electrode and the reference electrode.

An advantage of the use of a monitoring signal of known frequency thatis detected via analysis of received signals is that the monitoring maybe carried out continuously, whilst EMG signals are simultaneouslydetected and evaluated.

An index indicative of the quality of the electrode connections asmonitored over a period of time may be recorded, suitably providing asingle FIGURE which indicates to a user how good over the use period theconnection has been. The index figure may for instance be generated byproducing a numerical output indicative of the quality of connection atperiodic intervals and averaging the values of said numerical outputvalues over time.

In one aspect, the invention provides a novel approach to monitoringelectrode connections, such as electrode connections to the skin of anindividual. Accordingly, the present invention provides apparatus forreceiving and monitoring electrical signals comprising respective inputsfor receiving signals from at least a first electrode and a referenceelectrode, a Fourier transform analyser for extracting frequency contentinformation from said signals, a source of a monitoring signal having afrequency, an output for use in applying said monitoring signal to asaid electrode, and means for determining the amplitude of the saidfrequency of the monitoring signal in signals received at said inputs asdetermined by said Fourier transform analyser.

The Fourier transform analyser is preferably a DFT analyser, such as anFFT analyser. This can provide a power spectrum of the received signalsin which the amplitude of the signal at the frequency of the monitoringsignal can be determined.

In yet another aspect the invention relates to a method for amelioratingand/or treating undesired muscular activity, in particularly bruxism, byapplying electrodes as described above to the skin above a facial muscleof said individual, preferably the masseter muscle or the temporalismuscle, applying a stimulating signal, should a bruxism event happen,and monitoring the connection between the electrodes and the skin.Accordingly, in a further aspect the invention relates to a method formonitoring activity of the temporal muscle and/or the masseter muscle,in particular activity due to teeth and/or jaw clenching, said methodcomprising the steps of:

-   -   measuring EMG signals from said electrode(s) when the electrodes        are applied to the skin,    -   applying a monitoring signal having a frequency to said at least        first electrode,    -   determining the received signal of said frequency of the        monitoring signal in signals received, for monitoring the        quality of the electrical connection between the electrodes and        the skin    -   providing an electrical stimulation feedback signal upon the        detection of EMG signals indicative of a muscular activity of        the individual,        wherein the feedback signal is modulated if the quality of the        electrical connection between the electrodes is changed.        Embodiments of this aspect is described above in relation to the        apparatus.

Furthermore, the present invention provides a method for monitoring theelectrode connection, wherein the incoming signals relating to themonitoring signals are analysed by a Fourier transform analyser.Accordingly, in another aspect, the invention relates to a method formonitoring electrode connections, by receiving and monitoring electricalsignals from at least a first electrode and a reference electrode,extracting frequency content information from said signals andperforming a Fourier transform analysis, applying said monitoring signalto a said electrode, and determining the amplitude of the said frequencyof the monitoring signal in signals received as determined by saidFourier transform analyser. Embodiments of this aspect is describedabove in relation to the apparatus.

The invention will be further described and illustrated by reference tothe accompanying drawings in which:

DETAILED DESCRIPTION OF THE DRAWINGS

The circuitry shown in FIGS. 1 and 2 is that of apparatus for monitoringbruxism which is generally of the kind described in detail in WO2004/087258. The apparatus has an electrode assembly comprising a firstelectrode 10, a second electrode 14 and a reference electrode 12disposed between the first and second electrodes. The three electrodesare applied to the skin of a user above either the temporal muscle(musculus temporalis) or the masseter muscle (musculus masseter). EMGsignals received from the electrodes 10 and 14 are fed via an analogswitch, switching between sending and receiving functions as discussedbelow, to an Instrumentation amplifier and then to a notch filter forremoving ambient 50/60 Hz noise. After processing by a digital amplifierthe signals are low pass filtered with a cut off nominally at 500 Hzwhich allows a significant amount of the frequency content of themonitoring signal at 610 Hz to pass. The resulting filtered signals aresubjected to FFT analysis in a micro-processor to analyse theirfrequency content. The micro-processor further controls the switching ofthat analogue switch between receiving and sending states.

If there is found to be present a sufficient amplitude at one or morepreset frequencies or within one or more preset frequency bands in thereceived EMG signals, it is considered that an episode of bruxism isoccurring and an instruction is sent to the unit marked ‘stimulator’ togenerate a stimulation signal, which is applied to the referenceelectrode. This gives the user a mild electric shock which is designedto be insufficient to wake them if they are sleeping. Current passesfrom the reference electrode to each of the first and second electrodes.

The micro-processor also outputs trigger signals to a first output 16and to a second output 18 which connect respectively to first and secondmonitor signal generators DAC0 and DAC1 which convert the digitaltrigger signals to a 610 Hz sine wave monitoring signal. The monitoringsignals are thereby applied alternately to the first and secondelectrodes via a filter to dampen the signal and to exclude a DCcomponent (FIG. 2) at a voltage sufficient to achieve a preset currentvalue. The voltage is therefore dependent on the resistance between therelevant electrodes (electrode 10 to reference 12 in the case of DAC0and electrode 14 to reference 12 in the case of DAC1). As shown in FIG.2, the monitor signals are picked up from the reference electrode andare fed to an analogue to digital converter and are analysed in themicro-processor as part of the FFT analysis of the EMG signals receivedfrom the first and second electrodes.

Sufficient of the applied 610 Hz signal passes the filters to be pickedup in the EMG signals.

If the amplitude of the monitoring signal voltage applied to one (butnot the other) of electrodes 10 and 14 to achieve the preset current istoo high, implying a poor connection between the electrodes, thestimulation signal intensity is reduced, suitably by half, to avoidburning or skin irritation due to too great a current being passed fromthe reference electrode 12 to just the better connected one of the firstand second electrodes 10, 14. If the amplitude of the monitoring signalis too high as received from both of the electrodes 10, 14, then thegiving of the stimulation signal is blocked. This could be due to a poorconnection either at both of electrodes 10 and 14, or just at thereference electrode. It can happen that one or both of the first andsecond electrodes becomes short circuited to the reference electrode,for instance if conductive gel becomes smeared over the skin betweenthem. One will then detect too low an amplitude of the 610 Hz monitoringsignal.

A log is written in the apparatus for the purposes of informing the userthat one or both electrodes was not adequately connected during thesession.

In a set-up phase of the use of the apparatus, the monitoring signalsare used as a basis for confirming to the user (or not as appropriate)that an initial good connection of the electrode assembly has been made.

In this specification, unless expressly otherwise indicated, the word‘or’ is used in the sense of an operator that returns a true value wheneither or both of the stated conditions is met, as opposed to theoperator ‘exclusive or’ which requires that only one of the conditionsis met. The word ‘comprising’ is used in the sense of ‘including’ ratherthan in to mean ‘consisting of’.

1-35. (canceled)
 36. Apparatus comprising: an electrode assemblycomprising three electrodes: a first, a second and a referenceelectrode, mounted on a common substrate in a fixed spatial relationshipone to another wherein said reference electrode lies approximatelyequidistant from said first and second electrodes, said electrodeassembly adapted for being applied to the skin of an individual, whereinthe apparatus is operative to receive and monitor electrical signals andprovide electrical stimulation to said individual, said apparatusfurther comprising: inputs for receiving signals from said electrodes,means for measuring EMG signals from said electrode(s) when theelectrodes are applied to the skin, a source of a monitoring signalhaving a frequency and outputs for use in applying said monitoringsignal alternately to said first and second electrodes, means fordetermining the amplitude of the frequency of the monitoring signal atthe input of said reference electrode, means for providing an electricalstimulation feedback signal to the reference electrode upon thedetection of EMG signals indicative of a muscular activity relating tobruxism of the individual, wherein said apparatus is configured so thatsaid monitoring signal is applied at a voltage sufficient to produce apreset current between the first electrode and the reference electrodeand between the second electrode and the reference electrode, whereinthe amplitude of the frequency of the monitoring signal at saidreference electrode is measured to determine the quality of theconnection via the skin between the first electrode and the referenceelectrode and the quality of the connection via the skin between thesecond electrode and the reference electrode, and wherein the feedbacksignal is modulated if the quality of the electrical connection betweenthe electrodes is changed.
 37. Apparatus as claimed in claim 36,comprising a Fourier transform analyser for extracting frequency contentinformation from said received signals.
 38. Apparatus as claimed inclaim 36, further comprising means for measuring the amplitude withinthe received signals of at least one selected frequency other than thatof said monitoring signal.
 39. Apparatus as claimed in claim 37, whereinsaid apparatus is for measuring EMG signals from said electrodes whenthe electrodes are applied to the skin and the amplitude of the saidfrequency of the monitoring signal in signals received at said inputs asdetermined by said Fourier transform analyser is measured for monitoringthe quality of the electrical connection between the electrodes and theskin.
 40. Apparatus as claimed in claim 36, wherein said EMG signalsindicative of the muscular activity are identified by measuring thereinthe amplitude of one or more selected frequencies different from thefrequency of the monitoring signal.
 41. Apparatus as claimed in claim36, wherein the feedback signal is reduced when the quality of theelectrical connection is reduced.
 42. Apparatus according to claim 36,wherein the feedback signal is reduced if the amplitude of the saidfrequency of the monitoring signal in signals received at said inputs isnot below a preset threshold level or wherein the intensity of thefeedback signal is reduced if the amplitude of the said frequency of themonitoring signal in signals received at said inputs is not below apreset threshold level.
 43. Apparatus as claimed in claim 42, whereinthe feedback signal is reduced by half or blocked.
 44. A method formonitoring activity of the temporal muscle and/or the masseter muscle,in particular activity due to teeth and/or jaw clenching, said methodcomprising the steps of: providing an apparatus as defined in claim 1comprising electrode(s), measuring EMG signals from said electrode(s)when the electrodes are applied to the skin, applying a monitoringsignal having a frequency to said at least first electrode, determiningthe received signal of said frequency of the monitoring signal insignals received, for monitoring the quality of the electricalconnection between the electrodes and the skin, providing an electricalstimulation feedback signal upon the detection of EMG signals indicativeof a muscular activity of the individual, wherein the feedback signal ismodulated if the quality of the electrical connection between theelectrodes is changed.
 45. The method as claimed in claim 44, whereinthe receiving signal is determined by determining the amplitude of thesaid frequency of the monitoring signal in signals received.
 46. Themethod as claimed in claim 44, comprising extracting frequency contentinformation from said received signals and performing a Fouriertransform analysis, such as an FFT analysis.
 47. The method as claimedin claim 44, comprising for receiving signals from said first electrode,from a second electrode and from said reference electrode.
 48. Themethod as claimed in claim 47, wherein the respective electrodes areconnected.
 49. The method as claimed in claim 44, wherein saidelectrodes are provided in an electrode assembly in which said electrodeassembly comprises three electrodes mounted on a common substrate in afixed spatial relationship one to another.
 50. The method as claimed inclaim 44, wherein said reference electrode lies approximatelyequidistant from said first and second electrodes.
 51. The method asclaimed claim 44, further comprising the step of measuring the amplitudewithin the received signals of at least one selected frequency otherthan that of said monitoring signal.
 52. The method as claimed in claim44, wherein said activity is an episode of bruxism.
 53. The method asclaimed in claim 44, wherein said EMG signals indicative of the muscularactivity are identified by measuring therein the amplitude of one ormore selected frequencies different from the frequency of the monitoringsignal.
 54. The method as claimed in claim 44, wherein the feedbacksignal is reduced, when the quality of the electrical connection isreduced.
 55. The method according to claim 44, wherein the feedbacksignal is reduced if the amplitude of the said frequency of themonitoring signal in signals received is not below a preset thresholdlevel or wherein the intensity of the feedback signal is reduced if theamplitude of the said frequency of the monitoring signal in signalsreceived is not below a preset threshold level.
 56. The method asclaimed in claim 44, wherein the feedback signal is reduced by half orblocked.
 57. The method as claimed in claim 44, wherein said monitoringsignal is applied at a voltage sufficient to produce a preset currentbetween the first electrode and the reference electrode and between thesecond electrode and the reference electrode.